Many transition metal catalyst systems based on titanium, vanadium, chromium, molybdenum, palladium, and cobalt have been reported in the prior art for the preparation of syndiotactic 1,2-polybutadiene (see, e.g., J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press: New York, 1979, P. 144). However, the majority of these catalyst systems have no industrial application because they have insufficient polymerization activity and stereoselectivity and in some cases produce low molecular weight polymers or cross-linked polymers unsuitable for commercial use. The following cobalt-containing catalyst systems are well known for the preparation of syndiotactic 1,2-polybutadiene:
I. Cobalt dibromide/triisobutyl aluminum/water/triphenyl phosphine (Jap. Kokoku 44-32426, U.S. Pat. No. 4,182,813 (1/8/1980), assigned to Japan Synthetic Rubber Co. Ltd.) and
II. Cobalt tris(acetylacetonate)/triethyl aluminum/water/carbon disulfide (U.S. Pat. No. 3,778,424 (1970), Jap. Kokoku 72-19,892, 81-18,127, 74-17,666, 74-17,667; Jap. Kokai 81-88,408, 81-88,409. 81-88,410, 75-59,480, 75-121,380, 75-121,379, assigned to Ube Industries Ltd.).
These two catalyst systems also have serious disadvantages. The cobalt dibromide/triisobutyl aluminum/water/triphenyl phosphine system yields syndiotactic 1,2-polybutadiene having very low crystallinity. In addition, this catalyst system develops sufficient catalytic activity only in halogenated hydrocarbon solvents as polymerization medium, and halogenated solvents present the problems of toxicity. The cobalt tris(acetylacetonate)/triethyl aluminum/water/carbon disulfide system uses carbon disulfide as one of the catalyst components, thereby necessitating the use of special safety measures due to its high volatility, low flash point as well as toxicity. Furthermore, the syndiotactic 1,2-polybutadiene produced with this catalyst system has very high melting point (200-210.degree. C.) and is therefore difficult to process. Accordingly, many restrictions are required for the industrial utilization of the two said catalyst systems of the prior art.
Coordination catalysts based on chromium compounds such as triethylaluminum/chromium tris(acetylacetonate) have low activity and give rise to low molecular weight polymers and therefore have not been usable on a commercial scale. Japanese patents JP-A-7306939 and JP-A-7364178, both assigned to Mitsubishi, disclose a process for polymerization of 1,3-butadiene to amorphous 1,2-polybutadiene by using a ternary catalyst system comprising: (a) a soluble chromium compound, (b) a trialkyl aluminum compound, and (c) a dialkyl hydrogen phosphite. The product was reported to be a white rubbery polymer which contained a portion of gel and displayed no obvious melting point. U.S. Pat. No. 4,751,275, assigned to Bayer, discloses a process for the preparation of syndiotactic 1,2-polybutadiene by solution polymerization of 1,3-butadiene in a hydrocarbon polymerization medium. The catalyst used in this solution polymerization contains a chromium-III compound which is soluble in hydrocarbons, a trialkylaluminum compound, and dineopentyl phosphite or neopentylmethylphosphite. However, the polymerization product was not well characterized as neither the melting temperature nor the degree of syndiotacticity is reported.
U.S. Pat. No. 4,168,357 and U.S. Pat. No. 4,168,374, both assigned to Goodyear, describe chromium-containing catalysts for the prepartion of high cis-1,4-polypentadiene.
Notwithstanding the foregoing prior art, it would be advantageous to develop a new and improved catalyst system that can be used to produce a syndiotactic 1,2-polybutadiene product having a higher melting temperature and increased syndiotacticity compared to the syndiotactic 1,2-polybutadiene produced by the processes of the prior art.